This invention relates to certain melt-fusible polyimides, especially to those which can be melt processed without deleterious decomposition.
For the purpose of the present disclosure and claims, the term "melt-fusible" means that the material can be heated without significant decomposition above its glass transition temperature (Tg), if it is amorphous, or above its crystalline melting point (Tm), if it has crystallinity, and coalesced under pressure. The term "melt processible" means that the material can be fabricated by conventional melt processing techniques such as extrusion and injection molding, in which the melt passes through an orifice. While all the polyimides of the present invention are melt-fusible, the melt viscosity of some polyimides may be so high that they will not be readily melt processible. But even those high viscosity materials are capable of being formed into useful void-free articles by other techniques, such as, for example, fusion in situ on a support or in a heated mold under pressure.
Polyimides are condensation type polymers having a repeating unit of the type shown in Formula (A), below: ##STR1## where Z is a suitable tetravalent organic radical, which may be a simple structure such as that derived from the benzene ring or a more complex structure such as that derived from benzophenone, or any other appropriate, usually aromatic, tetravalent radical; and Q is a divalent organic radical.
One of the important industrial applications of polyimides is as binders for advanced composite materials, especially for use in the aerospace industry; e.g., in aircraft fuselages, wings, flight control surfaces, and missile nose cones, etc. The usual manner of making sheets of composite materials based on polyimides involves impregnation of a fibrous substrate, such as a woven or nonwoven fabric, with a solution of either the polyimide itself or one or more polyimide precursors and then either simply evaporating the solvent or forming a high molecular weight polyimide in situ. Such in situ polyimide formation, which usually is conducted at an elevated temperature, often is referred to as "curing". This expression will be used throughout the present disclosure in the same sense. The polyimide usually is made from the dianhydride of a suitable tetracarboxylic acid in one of two ways, as shown below in equations (1) and (2) for one pathway and (I), (II), and (III) for the other pathway ##STR2##
Thus, dianhydride (B) may be first converted by reaction with diamine H.sub.2 NQNH.sub.2 into polyamide acid (C), which then can be chemically or thermally dehydrated to polyimide (F). Alternatively, dianhydride (B) is first esterified with alcohol ROH (e.g., ethyl alcohol, R=C.sub.2 H.sub.5) to diester diacid (D), which forms with diamine H.sub.2 NQNH.sub.2 salt (E). This salt then is thermally cyclized to polyimide (F). Water and, in the appropriate case, alcohol liberated at high temperature are evaporated from the surface of the fibrous composite substrate, and the polyimide remains. The evaporation of reaction solvent, water, and alcohol can cause void formation because of vapor entrapment in the polymer mass. In order to eliminate the voids, it is expedient to compress the freshly made, hot composite to break up the gas bubbles and expel the gases. However, this is possible only when the polyimide can be heated without significant decomposition to a temperature at which it is sufficiently low in viscosity to respond effectively to such a treatment.
The most commonplace tetracarboxylic acid dianhydride used in the manufacture of polyimides is pyromellitic dianhydride, sometimes hereafter referred to as PMDA, represented by formula (G), below: ##STR3##
Another representative dianhydride used for this purpose is 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride (BTDA), formula (H), below: ##STR4##
A tetracarboxylic acid, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane, formula (I), below, has been used in some polyimides, ##STR5## which were made from the tetracarboxylic acid, a diamine, and a suitable solvent in one step by heating the well mixed ingredients to a sufficiently high temperature.
It was known in the past that the usual all aromatic PMDA-based polyimides were not melt-fusible because their crystalline melting points were well above the onset of thermal decomposition, which is about 450.degree. C. A crystalline, high molecular weight, infusible polyimide was formed with such diamines as m-phenylenediamine, p-phenylenediamine, 4,4'-oxydianiline (J), and 1,3-bis(4-aminophenoxy)benzene (K). ##STR6##
Fiber reinforced laminates based on the usual polyimides made from precursor solutions involving BTDA, formula (H), would normally have a high void content, which would not be readily eliminated because of crosslinking reactions of ketone carbonyl groups with amino groups. Because of this porosity, both the mechanical properties and the long term thermal-oxidative stability of the polyimides were adversely affected. A commercial product developed by Hughes Aircraft Company and sold under the name "Thermid" 600, could, however, be processed to a low void product. The uncured low molecular weight acetylene end-capped oligomer (formula L) could be converted to a high molecular weight product without the evolution of volatile by-products by means of the acetylene end-group coupling reactions. ##STR7## In spite of this advantage over other BTDA-based polyimides, this product was brittle and had low thermal-oxidative stability.
Polyimides based on the tetracarboxylic acid (I) could be processed to a low void product, which had excellent physical properties. However, the starting tetracarboxylic acid is quite expensive.
Polyimides based on PMDA and 2,2-bis[4-(4-aminophenoxy)phenyl]propane (M) ##STR8## have been reported independently by Sachindrapal et al., Makromol. Chem., Rapid Comm. 1, 667-670 (1980), and by Sazanov et al. Vysokomol. Soedin., (B), 20, 820-824 (1978), No. 11.
Polyimides based on the hexafluoroisopropylidene analog of diamine M and various dianhydrides also are disclosed in U.S. Pat. No. 4,111,906 to Jones et al. (TRW, Inc.), and preparation of two such polyimides is described in the examples. However, neither the above two publications nor the patent suggest the melt fusibility of any of the polyimides.
It thus appears very desirable to be able to produce polyimides based on pyromellitic dianhydride (or on pyromellitic acid), which would be melt-fusible, would have sufficiently low viscosity below their decomposition temperature to permit efficient working, especially removal of gas and voids, would have good physical properties in their cured form, and would have good oxidative stability.